1. Field
This document relates to a DC-DC converter and a controlling method thereof. In addition, this document relates to a display device which is driven by a power generated by the DC-DC converter and the controlling method thereof.
2. Related Art
Various kinds of electronic devices are required to be supplied with a stable direct current (“DC”) power, and thus have a DC-DC converter. The DC-DC converter receives a DC power to generate a DC output. The DC-DC converter, which is driven by a voltage controlled method, adjusts a duty ratio based on variation of a load and regulates the DC output.
In order to reduce the standby power in the electronic devices, the DC-DC converter has a burst mode function for reducing switching losses when the load is low. The burst mode operation in the DC-DC converter is as shown in FIGS. 1 and 2.
Referring to FIGS. 1 and 2, the DC-DC converter includes first to third operational amplifiers AMP1 to AMP3, a logic unit 10, a driving unit 11, and a transistor Q1.
The first operational amplifier AMP1 compares a feedback voltage input to a feedback terminal FB with a high potential reference voltage Vref_max, and generates an output indicating whether the feedback voltage is higher or lower than the reference voltage Vref_max. The feedback terminal FB is applied with an output voltage divided by a voltage division resistor circuit. The second operational amplifier AMP2 compares a feedback voltage Vfb with a low potential reference voltage Vref_min, and generates an output indicating whether the feedback voltage is higher or lower than the reference voltage Vref_min. The third operational amplifier AMP3 compares with each other voltages at both ends of a resistor R connected between the transistor Q1 and a ground GND so as to detect a current flowing through the transistor Q1, and outputs the detected result to the logic unit 10.
The logic unit 10 receives the outputs from the first and second operational amplifier AMP1 and AMP2, and repeatedly generates switch-on pulses until the feedback voltage Vfb increases to the high potential reference voltage Vref_max after reaching the low potential reference voltage Vref_min. In addition, the logic unit 10 stops generating the switch-on pulses until the feedback voltage Vfb decreases to the low potential reference voltage Vref_min after reaching the high potential reference voltage Vref_max. The driving unit 11 turns on the transistor Q1 in response to the switch-on pulses from the logic unit 10. In FIG. 2, the reference numeral “SW” denotes a voltage at a switch terminal SW which varies depending on the turn-on/turn-off of the transistor Q1. The output voltage Vout is a voltage developed passing through the switch terminal SW, a diode (not shown), and so on.
When the DC-DC converter having the burst mode function as shown in FIGS. 1 and 2 is employed as a power converter in a display device so as to reduce power consumption in the display device, ripples of driving voltages supplied to a display panel (corresponding to the load) in the burst mode are heighten, whereby a viewer can recognize that images displayed in the display panel shake.
If a full white load where all the pixels in an organic light emitting diode (OLED) display emit light at the maximum brightness is assumed to be 100%, when a grayscale for data input to the OLED display is adjusted to be 30% and the DC-DC converter provides an output voltage in the burst mode to the display panel of the display device as a high potential driving voltage VDD, an image displayed in the OLED display at this time is as shown in FIG. 3. In this case, the ripple cycles of the high potential driving voltage VDD in the burst mode are seen by a viewer, and thereby the viewer can clearly recognize a screen shaking phenomenon.
When a grayscale for data input to the OLED display is reduced to 10% and the DC-DC converter provides an output voltage in the burst mode to the display panel of the display device as the high potential driving voltage VDD, an image displayed in the OLED display is as shown in FIG. 4. In this case, the ripple cycles are hardly seen by a viewer, and thereby the viewer a little recognizes the screen shaking phenomenon.
FIGS. 5 to 7 are waveform diagrams illustrating examples of waveforms of which ripple amplitudes of output voltages are the same and frequencies are different from each other. The DC power output from the DC-DC converter has much influence on brightness level and brightness uniformity. Brightness variation in the display device caused by the difference between ripple amplitudes of the DC power output from the DC-DC converter is scarcely seen with naked eyes of a viewer. In contrast, brightness variation in the display device caused by the time difference between a switching duration and a non-switching duration can be seen with naked eyes of a viewer depending on ripple frequencies of the DC power. For example, if the ripple frequency of the DC power output from the DC-DC converter is very high as shown in FIG. 5, or is very low as shown in FIG. 7, a viewer hardly recognizes the screen shaking. However, the viewer can recognize the screen shaking in the ripple frequency of the DC power output in the burst mode (FIG. 6).
The burst mode in the DC-DC converter is not applied when the display device is operated in a light load. Therefore, it is necessary to use a voltage output from the DC-DC converter in the burst mode as the standby power and prevent degradation of a display quality when the display device is operated in the light load.